Interferometric Synthetic Aperture Radar (InSAR) & Differential InSAR (DInSAR)

Radar (RAdio Detection and Ranging) measures the strength and round-trip time of the microwave signals that are emitted by a radar antenna and reflected off a distant surface or object. The radar antenna alternately transmits and receives pulses at particular microwave wavelengths (typically in the range 1cm to 1m, which corresponds to a frequency range of about 300MHz to 30GHz) and polarisations (waves polarised in a single vertical or horizontal plane).

In the case of satelliteborne radar sensors, they measure the amount of energy which returns to the satellite after it interacts with the earth's surface, which is presented in the form of an intensity image, but also includes the round-trip time between the sensor and the surface, which is represented as the phase image. Unlike optical sensors, the radar's microwave energy penetrates clouds, rain, dust, or haze, and acquires images regardless of the sun's illumination, enabling radar to collect data under almost all atmospheric conditions.

A synthetic aperture radar, or SAR, is a coherent radar system that generates high resolution remote sensing imagery. Signal processing uses magnitude and phase of the received signals over successive pulses from elements of a 'synthetic aperture' to create an image.

Interferometric SAR (InSAR or IfSAR) systems exploit the phase difference from two SAR images acquired over the identical scene, thereby useful geodetic information such as the digital elevation model (DEM) can be derived. Two sets of SAR image values are registered by using correlation techniques in the InSAR processing software to an accuracy of 1/8 of a pixel. The phase differences between the two images are then calculated to generate the interferogram. These phase differences in the interferogram wrap around in cycles of 2pi, and need to be 'unwrapped' in order to obtain absolute phase values. Finally, the absolute phase can be converted into height information.

Based on InSAR, a new technique called differential InSAR (DInSAR) allows for the measurement of small deformations of the terrain (down to the millimetre level) that have occurred between the two different image acquisitions. In DInSAR analysis, the topographic contribution to the phase difference is carefully removed using a DEM. Atmospheric disturbances can be either accounted for using independent observations such as GPS, or neglected if the tropospheric delay is considered to be homogeneous at the time of the radar image acquisition. The DInSAR-based technique will complement current ground survey techniques and can be adopted and implemented on an operational basis to a wide range of applications such as the monitoring of: 

1. mine-induced subsidence,                                                
2. oil/gas field subsidence,
3. urban subsidence due to tunnel construction and/or groundwater extraction,
4. ice flow in the polar regions,                              
5. seismic deformation,
6. volcanic activity,                                                
7. landslide/mass movement,
8. organic soil subsidence, and                               
9. beach/coast erosion. 

DInSAR is an active research area (see the IAG's Sub-Commission 4.4 "Applications of Satellite & Airborne Imaging"). DInSAR research at UNSW was commenced by Dr. Linlin Ge in 2001, following research commenced by Prof. John Trinder into the use of the InSAR technique for determining Digital Elevation Models. In mid-2001 former-PhD student Diana Polonska commenced her studies on the feasibility of DInSAR for ground movement detection due to (pre-/post-)seismic activity.

An ARC-Discovery grant (2002-2004) "Integrated Space Geodetic Techniques for Ground Subsidence Monitoring Due to Underground Mining and Similar Activities" supported the ramping up of DInSAR research, in collaboration with A/Prof. Howard Zebker of the Standford University and Prof. Makoto Omura of the Kochi Women's University. In addition to Linlin Ge, research associate Dr. Eric Cheng and visiting fellow Dr. Makota Omura (from Japan) made crucial contributions during 2002. Also in 2002, an ACARP (Australian Coal Association Research Program) project "Monitoring Ground Subsidence Monitoring Using Integrated Space Geodetic Techniques" allowed research to commence on the effect of underground coal mining south of Sydney on ground subsidence.

In 2003 Michael Chang commenced his PhD studies, and the DInSAR group was supported by two visiting fellows from China. Additional collaborators and students carried out research in 2004: Tianzhen Yao, Zhao (Jackie) Jing, and Steven Song. In 2005 Wing Yip Lau commenced his research studies.

From 2004, the CRC for Spatial Information is supporting the Project 4.2 "Digital Elevation Model Generation & Differential Synthetic Radar Interferometry", for which Linlin Ge is project leader. An ARC-Linkage(International) grant (2004) "Integration of InSAR, GPS and GIS" supported international collaborative research on DInSAR between UNSW and the Hong Kong Polytechnic University (A/Prof. Xiaoli Ding). 

This web page provides a gateway to a number of InSAR-related projects being carried out at the School of Surveying & SIS, as well as providing links to some useful InSAR web sites.